Experimental Constraints on the Rheology and Seismicity of Subducting Lithosphere and the Slab-Wedge Interface

为了了解俯冲带地震的时空分布，以及导致板/楔界面断层滑移行为的广谱过程，必须约束蚀变岩石圈的流变特性、它们在脱水反应过程中的演化过程、反应产物的流变特性以及变质反应与下沉板块输运特性之间的反馈。目前的地幔楔角流模型主要受俯冲带表面热流测量的影响，使用各种边界条件来施加俯冲板块与上覆地幔楔之间的解耦。一种假设是，在板块上方的一层弱蛇纹岩（一种水合镁硅酸盐）容纳了大部分的变形。蛇纹岩也被假设在许多俯冲环境中观察到的双地震带的起源中起重要作用，并在沿板块的中深地震活动的起源中起重要作用。在其他情况下，人们假设钙铝硅酸盐（一种水合钙铝硅酸盐）脱水会促进地震活动性。在这种情况下，地震活动性被解释为由于脱水过程中产生的高孔隙流体压力，以及随后的岩石脆化。在广泛的滑动速度范围内检查断层材料的摩擦行为对于理解地震期间应力演化和滑动的动力学至关重要。地球物理观测表明，片状构造矿物（如蛇纹石、滑石和其他粘土）可能控制着断层爬行段的强度和摩擦行为。先前对蛇纹岩的摩擦实验记录了板块构造位移速率下的速度强化行为，与这一假设相一致。然而，蛇纹岩的高速摩擦实验表明，在~ 0.1 m/s以上的滑移速度下，蛇纹岩明显减弱和脱水。我们的实验表明，在特征减弱速度Vw ~ 0.1 m/s以上，摩擦对速度的依赖约为1/V，这与闪速加热和随后的粗糙（不均匀）接触减弱的理论预测一致。将这些结果外推到地幔条件表明，缓慢滑动事件和/或缓慢地震可能由地震破裂形成蛇纹岩区。主要是由于我们实验室的实验结果对这些假设提出了挑战和信息，我们建议研究(a)反长花岗岩在高压和高温下的流变特性；(b)脱水反应动力学对蛇纹岩和蓝片岩力学演化的影响；(c)适合中深度地震条件下板坯脱水反应产物的流变学和(d)蛇纹岩在地震和次地震滑动速率下的摩擦特性。该项目的部分内容将成为布朗大学（Brown University）一名学生博士论文的基础，以及海伦·道尔（Helen Doyle）和另一名本科生的毕业论文章节。实验结果将为广大科学家提供约束条件，这些科学家对变质作用、地球动力学、地震学、地震物理学和流体输送等不同领域感兴趣，跨越了新的地球科学倡议之间的界限。我们将提供所有结果，包括所有原始数据文件的数字记录-通过在线补充和我们的实验室网站。

To understand the spatial and temporal distribution of earthquakes in subduction zones, as well as the processes responsible for the wide spectrum of fault slip behaviors on the slab/wedge interface, it is important to constrain the rheological properties of altered lithosphere, how they evolve during dehydration reactions, the rheology of reaction products, and the feedbacks between metamorphic reactions and transport properties of the downgoing slab. Primarily motivated by surface heat flow measurements in subduction zones, current mantle wedge corner flow models use various boundary conditions to impose decoupling between the subducting slab and overlying mantle wedge. One hypothesis is that a layer of weak serpentinite (a hydrated magnesian silicate) above the slab accommodates the majority of the deformation. Serpentine has also been hypothesized to play an important role in the origin of the double seismic zone observed in numerous subduction settings, and on the origin of intermediate and deep seismicity along the slab. In other settings, dehydration of lawsonite (a hydrated calcium-aluminum silicate) has been hypothesized to promote seismicity. In this case, seismicity is interpreted to result from high pore fluid pressures that arise during dehydration, and subsequent embrittlement of the rock. Examining the frictional behavior of fault materials over a wide range of sliding velocities is essential for understanding the dynamics of stress evolution and slip during earthquakes. Geophysical observations suggest that sheet-structure minerals (e.g., serpentine, talc and other clays) may control the strength and frictional behavior of creeping sections of faults. Previous friction experiments on serpentinite document velocity-strengthening behavior at plate-tectonic displacement rates, consistent with this hypothesis. However, high velocity friction experiments on serpentinite document dramatic weakening and dehydration at slip velocities above ~ 0.1 m/s. Our experiments demonstrate an approximately 1/V dependence of friction on velocity above a characteristic weakening velocity Vw ~ 0.1 m/s, consistent with theoretical predictions for flash heating and subsequent weakening of asperity (uneven) contacts. Extrapolation of these results to mantle conditions suggests that slow-slip events and/or slow earthquakes could be nucleated by seismic ruptures into serpentinized regions. Primarily motivated by new results from experiments in our lab that both challenge and inform some of these hypotheses, we propose to study (a) the rheological properties of antigorite at high pressure and temperature; (b) the role of dehydration reaction kinetics on the mechanical evolution of serpentinites and blue-schists; (c) the rheology of slab dehydration reaction products at conditions appropriate for intermediate depth earthquakes and (d) the frictional properties of serpentinites at seismic and infra-seismic slip rates.Parts of the project will form the basis of a Ph.D. thesis for a Brown University student and the senior thesis chapter for Helen Doyle and another undergraduate student. The results of the experiments will provide constraints for a wide range of scientists with interests in the diverse fields of metamorphism, geodynamics, seismology, the physics of earthquakes, and fluid transport, crossing the boundaries between the new GEOPRISMS initiatives. We will make all of our results available including digital records of all raw data files - via on-line supplements and our lab websites.